Water quality characteristics of Vanderkloof Dam and its potential for rainbow trout farming

Abstract

Vanderkloof Dam was periodically sampled between November 2014 and April 2016 for a range of water quality parameters to assess the potential for  cage culture of rainbow trout, Oncorhyncus mykiss. The dam is strongly stratified in summer and autumn, although upper water column temperatures  remain relatively cool, largely <25°C. The dam is considered oligotrophic, characterised by phosphorus limitation of primary production, particularly  during summer. Light is strongly attenuated in the epilimnion and heterotrophic community metabolism is observed through much of the water column.  Surface nutrients are depleted through the summer with phytoplankton assemblages dominated by the green algae, Oocystis lacustris, Desmodesmus  bicaudatus and Coelastrum microporum. The water column turns over during the winter and the nearly isothermal impoundment approaches 11°C.  Nutrients are re-introduced into surface waters following winter mixing coincident with an increase in diatoms. The preferred environmental window for  rainbow trout becomes severely contracted during the summer in that epilimnion temperatures exceed 21°C and thermocline/hypolimnion oxygen  concentrations fall below 3 mg·L⁻¹. The coincidence of relatively high pH >9.5 could exacerbate these physiological challenges. Hydrological conditions  for fish farming are most suitable in the proximity of the dam wall as opposed to further upstream. Here the hypoxic conditions that develop in the  thermocline/hypolimnion during summer and autumn are less pronounced, particularly towards the southern bank, where concentrations of total  suspended solids are generally <10 mg·L⁻¹ and water clarity is more favourable for visual feeders such as fish. Although rainbow trout may survive the  adverse conditions prevalent during the summer, growth is likely to be compromised and susceptibility to disease may increase.